How to explain Period 3 across the periodic table

Groups run down the periodic table; periods run across it. Once you have the groups sorted, the next SPM skill is explaining what changes as you move across a period, and Period 3 is the one the syllabus uses, because its eight elements show every trend cleanly. This guide walks across Period 3 of the periodic table from sodium to argon and explains each change with its reason, which is what earns the marks.

The eight elements of Period 3

Period 3 runs: sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl) and argon (Ar). Their proton numbers go from 11 up to 18, increasing by exactly one at each step.

The single most useful fact is what stays the same and what changes. Every Period 3 element has three occupied electron shells, that is what makes it Period 3. What changes across the period is the number of valence electrons, which climbs from 1 (in sodium, 2.8.1) to 8 (in argon, 2.8.8). Hold those two facts together and the trends follow.

Trend 1: valence electrons increase

From left to right the number of outer-shell electrons increases one at a time: Na has 1, Mg 2, Al 3, Si 4, P 5, S 6, Cl 7, Ar 8. This directly sets each element’s group, and it is the root cause of the chemical changes further down this list.

Trend 2: atomic size decreases

This one surprises students, because atoms get smaller across a period even though electrons are being added. The reason is the nucleus. Across Period 3 the proton number rises from 11 to 18, so the positive nuclear charge increases. Meanwhile the added electrons all go into the same third shell, so the shielding by inner shells stays about the same. The stronger nucleus therefore pulls the outer shell in more tightly, and the atom gets smaller from sodium to chlorine.

Trend 3: electronegativity increases

Electronegativity is the ability of an atom to attract the shared electrons in a bond. Across Period 3 it increases, for the same reason the atoms shrink: a larger nuclear charge acting on an outer shell that is not much better shielded pulls bonding electrons more strongly. So chlorine is much more electronegative than sodium. (Argon is left out of this trend because, as a noble gas, it does not normally form bonds.)

Trend 4: metal to non-metal

This is the change you can almost see. Across Period 3 the elements shift from metallic to non-metallic character:

• Sodium, magnesium and aluminium are metals, shiny, conducting solids that tend to lose electrons.
• Silicon is a metalloid (semi-metal) with in-between properties.
• Phosphorus, sulfur and chlorine are non-metals that tend to gain or share electrons.
• Argon is a noble gas, unreactive with a full outer shell.

So metallic character decreases and non-metallic character increases across the period. This links straight back to the valence electrons: elements on the left have few outer electrons and lose them easily (metals), while elements on the right have nearly full shells and gain electrons (non-metals).

Trend 5: oxides change from basic to acidic

This is a favourite Paper 2 trend, and it ties everything together. As you cross Period 3, the oxides of the elements change in character:

• Sodium oxide (Na₂O) and magnesium oxide (MgO) are basic oxides, metal oxides that react with acids.
• Aluminium oxide (Al₂O₃) is amphoteric, it reacts with both acids and alkalis.
• Silicon dioxide (SiO₂), phosphorus oxide (P₄O₁₀), sulfur oxides (SO₂/SO₃) and the chlorine oxide (Cl₂O₇) are acidic oxides, non-metal oxides that react with alkalis.

The pattern, basic on the left, amphoteric in the middle, acidic on the right, mirrors the metal-to-non-metal change, because metal oxides are basic and non-metal oxides are acidic.

Across versus down: don’t mix them up

The most common exam error is muddling “across” with “down”. Keep them separate:

• Across Period 3: valence electrons increase, atomic size decreases, electronegativity increases, metal changes to non-metal. The number of shells stays the same.
• Down a group: valence electrons stay the same, atomic size increases, the number of shells increases.

If you can state which direction a trend belongs to and why, you have the topic. You can trace Period 3 element by element on our interactive periodic table, checking each electron arrangement against its position, and revisit the electron-arrangement basics in matter and the atomic structure if the shell counting still feels shaky.

Where students lose easy marks

Period 3 is a generous topic because the answers are patterns, not calculations, but students lose marks by stating a trend without its cause, or by giving the “down a group” reason for an “across a period” question. If the two directions keep blurring, one focused session usually fixes it. Our online one-to-one lessons run in English from RM50 an hour with a paid one-hour trial; see how it works if you would like Period 3 taught against your own past-paper answers.